1. Field of the Invention
The present invention relates to the deposition of material layers onto semiconductor substrates by sputtering. More particularly, the present invention relates to the deposition of material layers by sputtering wherein the surfaces of any apertures extending into the uppermost surface of the substrate, such as holes or trenches, must be conformally covered by the material layer deposited on the substrate, and the thickness of the from layer deposited on the side walls of each of the holes or trenches, and the thickness of the deposition layer formed on the base of each of the holes or trenches, should be substantially equal at each location on the substrate.
2. Background of the Invention
Sputtering is one well known method of depositing a film layer on a semiconductor substrate. A typical sputtering apparatus includes a target and a substrate support pedestal enclosed in a vacuum chamber. The target is typically affixed to the top of the chamber, but is electrically isolated from the chamber walls. A voltage source maintains the target at a negative voltage with respect to the walls of the- chamber, thereby exciting a gas, which is maintained in the chamber at a low pressure, into a plasma. Ions from this plasma sputter the target.
As a first order approximation, the trajectories of the particles sputtered from any point on the target have a cosine angular distribution; that is, the density of sputtered particles ejected from a point on the target along a trajectory having a given angle from perpendicular to the target is proportional to the cosine of such angle. The target particles sputtered from the target generally travel in a straight line path and will tend to deposit on any surface that they contact.
One application of sputtering is to provide a conformal metal deposition layer on the surfaces of holes or trenches extending through one or more metal, dielectric or semiconducting film layers on the uppermost surface of the substrate. The metal deposited on the substrate by sputtering the target must form a continuous, i.e., conformal, coating on the wall and base of the holes or trenches.
To ensure that all conductors deposited on a substrate have the same resistivity, the metal must be deposited with a uniform thickness. Ideally, the thickness of the metal film layer deposited on the base of a hole or trench should be no different for holes near the periphery of the substrate than for holes near the center of the substrate. Furthermore, the thickness of the film layer deposited on the side wall of each of the holes or trenches should be symmetrical, and the film layer should also have a relatively uniform thickness over the entire height of the wall or span of the base of each hole or trench. Moreover, the film layer formed on the side wall of any of the holes or trenches should have the same thickness and symmetry as that formed on the sidewall of every other hole or trench. However, the film layer formed on the base of the hole or wench may be of a different thickness than the film layer formed on the wall of the hole or trench.
The uniformity of the film layer deposited on the wall and base of each hole or trench is dependent on the angular distribution of the trajectories of the individual particles of target material reaching each of the holes or trenches. Particles travelling in paths that are substantially perpendicular to the substrate surface will pass through the open end of the hole or trench and deposit on the hole or trench base. Particles travelling at angles from perpendicular to the substrate surface will typically deposit on the hole wall and on the upper surface of the substrate.
One common problem in sputtering systems is that asymmetric side wall coverage occurs in holes adjacent to the edge of the substrate. At an edge location, the quantity of target material reaching the hole from regions of the target located inwardly of the hole location exceeds the quantity of target material from the target regions located outwardly of the hole location. As a result, the film layer formed on the hole wall will be thickest on the outward side of the hole and thinnest at the inward side of the hole.
An additional problem with sputtering systems is the formation of voids when attempting to fill holes near the center of the substrate surface. At this location, the peripheral region of the target supplies a large quantity of particles which are travelling at highly oblique angles with respect to the surface of the substrate. These particles will deposit on the upper end of the hole walls and the surface of the substrate adjacent to the hole. The deposit formed from these particles will overhang the hole opening and thereby block the access of other target particles to the base of the hole and the lower side walls of the hole adjacent to the base of the hole. As a result, an overhanging deposition layer forms over the hole opening, and a non-filled region (i.e., a "void") remains within the hole.
A conventional solution to these problems is to provide a perforated collimator plate between the target and the substrate. The perforations have a ratio of length to diameter sufficient to screen out (i.e., block) target particles travelling in paths which are substantially oblique to the upper surface of the substrate. The screening is provided because the obliquely travelling particles collide with, and deposit on, the walls of the collimator holes. Because the portion of the target particle flux travelling obliquely to the substrate surface is screened out, the flux reaching each location on the substrate is contributed only by the portion of the target directly above the hole in the substrate, i.e., it is provided through the hole in the plate collimator directly above each substrate location. This provides a symmetrical flux at each location on the substrate reduces the deposition of overhanging material which can create un-filled voids within the holes, and thereby facilitates uniform, symmetrical filling of the holes with the target material.
However, the plate collimator has several disadvantages. First, a substantial quantity of the particles sputtered from the target are wasted by being deposited on the surfaces of the collimator. Typically at least 50%, and in some applications as much as 90%, of the material which would otherwise reach the substrate is screened out by the collimator. Additionally, the collimator holes quickly fill with deposited particles, which particles block the holes and reduce the hole width. Eventually the particles will substantially block or completely fill the holes and the plate collimator must be cleaned or replaced, thereby incurring costly idle time of the equipment during maintainence as well as the cost of the replacement pans. Finally, target particles may collide with the underside of the plate collimator at low energies, thereby forming loosely attached deposits which can fall onto, and thereby contaminate, a substrate.
Therefore, a need exists for a sputtering apparatus capable of depositing a uniform, symmetrical, conformal film in holes or trenches as well as on the surface of the substrate, with minimal maintenance requirements and minimal contamination of the substrate.